Generally, in timing chains in use for driving valve actuating devices for internal combustion engines, cam shaft chains for drivingly connecting the cam shafts, and balancer chains, hydraulic tensioners have been used on the slack sides of the chains in order to take up slacks of the chains that occur during operation of the chains and to generate tension in the chains.
A hydraulic tensioner of prior art typically includes a housing, and a plunger that is slidably received in the plunger bore formed in the housing. Oil pressure from the external source of pressurized fluid is supplied to the hydraulic chamber that is defined by the inner wall surfaces of the piston bore and the rear end surface of the piston.
In such a hydraulic tensioner, when the tension in the chain has increased due to resonance of a chain span and the chain imparts an excessive force to the plunger, oil pressure in the hydraulic chamber increases and an excessive reaction force acts upon the chain from the plunger. As a result, friction of the chain may increase.
Therefore, a hydraulic tensioner having a pressure relief valve has been proposed such that when the pressure in the hydraulic chamber has exceeded the predetermined level oil in the oil chamber is allowed to exit the oil chamber to restrain an increase in the pressure of the hydraulic chamber.
Hydraulic tensioners having pressure relief valves of prior art are shown in Japanese patent application publication Nos. 9-303506 and 2001-21013 (hereinafter referred to JP reference Nos. 9-303506 and 2001-21013, respectively). JP reference No. 9-303506 describes a disk-shaped pressure relief valve to open and close the oil passage formed at the distal end of the plunger. The stepped portion of the oil passage has a seat formed thereon and the pressure relief valve is biased against the seat, i.e. toward closure of the oil passage, by the elastic resilient force of the spring provided in the oil passage. JP reference No. 2001-21013 describes a ball as a pressure relief valve openably and closably operatable in the aperture of the valve housing. The aperture has a seat formed thereon and the ball is biased against the seat, i.e. toward closure of the aperture, by the elastic resilient force of the spring provided in the valve housing.
In such a hydraulic tensioner with a pressure relief valve, when the pressure in the hydraulic chamber has exceeded the predetermined level, the pressure relief valve leaves the seat against the force of the spring, thereby opening the pressure relief valve to discharge oil in the hydraulic chamber to the outside of the tensioner. As a result of this, oil pressure in the hydraulic chamber can be regulated.
However, the prior-art hydraulic tensioner is constructed such that the pressure relief valve opens only when the pressure in the hydraulic chamber has exceeded the predetermined level and it is thus difficult to correspond to the various operating conditions of the engine.
For example, in the case of a tensioner where a relatively high oil pressure is supplied in order to control chain motions at idle of the engine, pressure in the hydraulic chamber becomes higher than the expected value at the normal operating conditions of the engine and chain frictions thus increase. In order to solve such problem, the relief valve is set to open only at the normal operating conditions. However, in this case, at the time of resonance of a chain span, pressure relief of the hydraulic chamber cannot be fully carried out, thus causing a problem of an increase of chain friction due to excessive tension of the chain span.
Also, in the case of a tensioner where a relief valve is set to open only at the time of resonance of a chain span, pressure of the hydraulic chamber cannot be maintained at an appropriate value both at idle and at normal operating conditions. Similarly, a problem of an increase of chain friction occurs.
An increase of chain friction incurs a decrease of fuel efficiency of an engine, which is contrary to the demand for improved fuel efficiency in the automotive industry that has reflected recent environmental issues.
The present invention has been made in view of the above-mentioned circumstances, and its object is to prevent an increase of the chain friction in the various engine operating conditions by varying at multi-stages the oil pressure in the hydraulic chamber of a hydraulic tensioner having a pressure relief valve.